figuring out if first or last child

I have a structure that, simplified, looks like this:

<newsletter>
  <article></article>
  <article></article>
  <article></article>
</newsletter>

when I'm processing the <article>s, I need to do something different with
the first and last, that is:

<xsl:if test="[this is the first article]">...
and possibly for another issue
<xsl:if test="[this is the last article]">...

I can't figure out how I'm supposed to express this. Clarification in
novice terms would be great; I'm a beginner.  Thanks. Eric

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